Method and apparatus for forming hollow bodies of double or variable curvature



1958 v. A. MILLER EI'AL 5 Sheets-Sheet 1 METHOD AND APPARAT US FORFORMING HOLLOW BODIE OF DOUBLE 0R VARIABLE CURVATURE Filed July 1, 1952m H R mi win m J T E L i V A BY LEONARD FSHEPARD.

ATTORNEYJ.

Nov. 25, 1958 v. A. MILLER ETAL 2,861,296

PARATUS FOR' FOR gg BODIES METHOD AND AP MING HOLL OR VARIABLE CURVAT OFDOUBLE 5 Sheets-Sheet 2 Filed July 1, 1952 INVENTORS. VEELEAM/ L L E)?-A L BERT J HETH. BY LEONARD FSHEPAED.

A TTORNEVJ.

v. A. MILLER ETAL 2,861,296 APPARATUS FOR FORMING HOLLOW BODIES DOUBLEOR, VARIABLE CURVATURE 3 Sheets-Sheet 3 Nov. 25, 1958 METHOD AND OFFiled July 1, 1952 IN VEN TORS. V'sELE/LM/ LLEE. ALBEETJ HETH. BY LEO NA20 ESHEPA ED.

ATTOfi/VEKi United States Patent'O METHOD AND APPARATUS FOR FORMING HOL-LOW BODIES OF DOUBLE R VARIABLE CUR- VATURE Verie A. Miller, Albert J.Firth, and Leonard F. Shepard,

Dover, Del., assignors to International Latex Corporation, Dover, Dei.,a corporation of Delaware Application July 1, 1952, Serial No. 296,618

Claims. (Cl. 1819) This invention relates in general to the art offorming hollow bodies of double or variable curvature from thermoplasticsheet material; and more especially to the art of forming hollow bodiesfree from substantial optical distortion throughout a major areathereof.

The invention is especially useful for forming face pieces or panels foruse in airtight helmets of the character disclosed in Andrews et a1.application Serial No. 257,002, filed November 19, 1951. Such facepieces are convex both longitudinally and transversely and taper from awider end of relatively large radius of curvature, corresponding to theforehead portion to a narrower rounded end of substantially sharperradius of curvature corresponding to the chin portion and, therefore,the hollow body comprises a surface having varying degrees of transversecurvature throughout its length and a longitudinal curvature whichvaries in a transverse direction. Such face pieces are made of laminatedsheet material, such as Plexiglas, and the formation of such sheetmaterial into a complex curved surface has presented a very greatproblem, especially since such face pieces must be free of opticaldistortion, and if the face pieces as molded from the sheet material areoptically distorted costly and time consuming grinding and polishing isrequired. Additionally, it is desired that such face pieces be providedwith an electrical resistance grid for heating the panel. For example,such a grid may be formed of nichrome wire of a diameter of the order of0.003" and the wires must be maintained in parallelism and preferablyspaced apart equidistantly to prevent eyestrain or optical distortion.Furthermore, in view of the fineness of these wires it is necessary toprevent undue strains in the portion of the face piece containing theresistance wire grid in order to prevent breakage of the wires as wellas their displacement from parallelism.

An object of this invention is to provide improved apparatus and methodof forming or molding articles of the character described wherein thearticles so molded are free from substantial optical distortion.

Another object of the invention is to provide apparatus which is simple,durable and inexpensive to make.

Another object is to provide a method of molding hollow plastic bodiesfrom sheet material while retaining a substantial surface area of saidmaterial free from localized stresses.-

Another object is to produce face pieces of the character describedcontaining an electrical resistance wire disposed in parallel gridformation without breaking said wire or distorting said parallelism.

Other features and advantages of the invention will more fully appearfrom the following description, taken in connection with the drawings,showing the presently preferred apparatus for carrying out the method,in which:

Fig. 1 is a plan view of a preferred form of apparatus in accordancewith this invention;

Fig. 2 is a side elevation of the apparatus shown in F Figs. 3, 4 and 5are cross-sectional views taken along the lines 3-3. 4-4 and 55respectively of Fig; 1; and

Figs. 6, 7 and 8 are partial longitudinal sectional views taken alongthe longitudinal center line of Fig. 1 showing the position of the partsin various stages of operation.

The apparatus comprises the main units identified and designated ingeneral as follows:

A is a male form whose convex forming surface corresponds to theinterior surface of the body to be formed thereon;

B is a female form in the general shape of an arch or saddle whichtransversely spans the male form A adjacent one end for a limited extentin a longitudinal direction of the male form;

C is a wiping member;

D is a tuck-in member; and

E is a suitable support, such as a base plate, on which the foregoingparts are mounted in operative relationship.

Male form A The male form A provides the contour desired for the hollowbody to be formed or molded. In the instant embodiment this body is aface panel for a helmet having a double curved surface, i. e., a surfacewhich is curved longitudinally and transversely. Such face panels mustbe free of optical distortion and in order to avoid the necessity ofgrinding or polishing the surface the form A is made with a surfacehaving almost a glass-like finish. Preferably we employ for this purposea cast resin form made of material such as Rezolin casting resin whichis sufliciently durable and which does not mar the surface of the sheetblank when formed incontact with the male form or mold. 7

Referring more particularly to Figs. 1, 3 and 6: the form A at itslarger, rear, end is provided with a portion extending longitudinallyfor a distance designated L1 which in cross-section (Fig. 3) is ofsubstantially uniform curvature, such as the surface of a cylinderhaving a radius r whose center is disposed somewhat above thetransversely extending base 1 which is securely fastened to the baseplate E. The forward end of the form A gradually tapers throughout theremainder of its length L2 and is curved longitudinally and laterally,terminating in a somewhat rounded small end in plan view. Commencing atthe smaller end, which corresponds to the chain portion of the facepanel, the form A is undercut rearwardly toward and merges with the base1 so as to provide a relief recess, for the purposes to be laterdescribed. In the present embodiment this undercutting extends entirelybeneath the form 1 but it is understood that a groove of suitable depthwould be suificient for the purpose. The top of the undercut recesscorresponds in general to the rear edge of the face piece and theforward edge of the cylindrical zone L1 corresponds to the top of theface piece. A pair of locating pins, such as 3,

have their lower ends securely anchored in the form A with their upperends extending vertically upward from the plane of the base plate. Thepins 3 are preferably spaced about /8" from the rear or outer edge ofthe cylindrical zone L1 and are spaced apart about 478" along the arm ofthe surface, i. e., each about 2 from the center. In connection with thespacing of these pins, it is noted that when the pins were spaced thedistance of the order of A from the center line the results wereunsatisfactory and when moved outwardly a distance of the order of 2"from the center line operative conditions were found to be satisfactory.With the pins spaced at the smaller distance undue stress was noted inthe plastic blank being formed and this caused undesirable stresses andundesirable breakage of the resistance wires embedded therein.

It is here noted that the cylindrical zone Ll corresponds to thatportion of the blank being formed lying beyond the adjacent end of theface piece, and therefore is trimmed off after the molding or forming.is completed. This will be seen from Figs. End 7 in which the brokenline P indicates the trim line of the finished face piece. This extendedend of the blank which is to .be tri'mmed off is utilized for clampingengagement of methane withthe blank of sheet material being operatedupon.

The forward face of the saddle form B is inclined downwardly andforwardly, as at 6, and terminates in oppositely disposed legs, such as7 (Fig. 2), which are preferably provided with' metal plates,such as 8.Parallel guide pins, suchas 9, have their lower ends'fixed in the baseplate E, which at this point ispreferably reinforced by a plate 10,

and extend vertically upwardlythrough corresponding holesprovided'in'the saddle form B; to thereoy permit operative movement ofthe'form B relative to the form Awhile holdingthese form membersinregistered alignment. Suitable means is provided forclamping thefemale saddle'for m B in its downward position, such as a pair of clamps1'1 which are fixed to the base plate E'by means of a pedestal, such as12. It will be understood that the saddle form 13 serves to initiate thetransverse bending of the sheet blank to be formed by initially bendingthe'blarikinto'a relatively uniform curved surface, and in 'the'laterpart :of its operation serves to'firmly clamp the partially'formed blankthroughout its transverse arcuate extent in the zone L1.

Wiping member C The wiping memberC (Figs. 1 and 6) comprises a wipingplate 14 whose forward edge is provided with a concave recess 15conforming to the contour of the tapered end of the male formAthroughout the extent forwardly of the saddle member B at the maximumdiameter of said tapered end in plan view (Fig. l) The recess portion 15being of a dimension to extend from the plates 8, around the chinportion of the form A and of a size to provide a space betweenthe-concavity 15 and the male form A substantially equal to thethickness of the sheet blank'being operated upon; except that at thebottom of the recess there is a somewhat greater clearance. The plate 14is preferably'provided with oppositely disposed thickened strips, suchas' 16, which may be conveniently made of wood, intimately secured totheupper face of the plate 14 and provided therebetween aI-guideway '17.The wiping member C is pivotally mounted, as by hinges 18, to a panel 19which is slidably disposed on vertically extending guide pins, such as20, rigidly mounted on the base plate E. If desired, the guide pins Zt)may 'be omitted and the'panel 19 may be formed as an integral base plateE,as will be more fully pointed out below. In any event the position ofthe pivot points of the hinges 18 is such that when the wiping member Cis disposedin its lowermostposition the free edge liesclosely adjacent,and preferably in engagement with theinclined platesS on the adjacentface of the saddle member B; and the recessed edge of the plate 14 liesin the spaced relationabove referred to. Suitable clamping means, suchas a pair of oppositely disposed clamps '21, is provided for loclingthewiping member C in its lowermost position. i

- T uck-in member D The tuck-in member D (Figs. 1 and 8) is utilized togive the final shape to the chin section and comprises a forward end 24whose free edge is provided with a recess 25 of generally arcuate shapeconforming to the rounded forward end of the form A and slightly smallerthan the outline-of said form in plan view. This forward end is ofa'tliickness somewhat less than the height of the clearance recessprovided by the cut-away 2 and in the present embodiment'ismade of aplate, such as aluminum, approximately at" in thickness to providesuitable strength and which at'the concavity 25 is sharpened, from aboutthe arcuate zone 25a, to an edge about in radius disposed closely "tothe lower surface of the end 24. The tuck-in member D also comprises aforwardly extending body portion 26 whose outer end is provided with atongue 27 of reduced width received in a slot 28 provided in a handlever 29 which ispivotally'secured at 30 to the base plate It willbefnotedthat the free end 24 of the tuck-in member D'when in operativeposition lies in the groove '17 and is disposedon'theupper surface ofthe wiping plate 14, serving to guide {the vtuck-in member in a planeparallel to the'baseIplate E and generally parallel to the base 1 of themale form A as the tuck-in member is moved forwardly by moving'thehandl'ever 29 in an anti-clockwise direction about its pivot 30.

Before describing the method further details of the sheet material fromwhich the article is preferably formed will be referred to.'Sheet'material approximately 0.125" thick comprising laminatesof'suitable transparent plastic materialgsuch for example as poly/methylmethacrylate (known under the trade name Plexiglas and Lucite),

vinyl acetate (such as Vinylite) etc., or a combination of two or more'of'these materials, has been found suitable. For example, we have foundthe following specific laminated isheermaterial'tob entirelysatisfactory, i. e., a laminated sh'eet consisting of a three-plylaminate comi'p'risingt'wo plies 'e'ach- 2/ thick'Ple'xiglas and anintermediate 'vinylite ly 0.010" containing the heating elemerit"made'ot 0.003" nichrom wit-e arranged in the required parallel gridpattern. This grid pattern extends over an area of the sheetcorresponding to that area of the'face piece to be disposed in front ofthe eyes of the wearer throughout the desired field of vision. It is to'be understood'that the in'ventio'n'is not limited to bodies made fromlaminated material or containing heating wires.

Method A blank of a desired'sheet material is selectedand for theparticular purposes of making'face pieces of suitable size this blankwill be approximately 12" square. Holes are drilled in the blank'toreceive the positioning pins 3-'3 (Figs. 1 and-6) and'in this connectionit-is noted that wehave obtained best results by making theseholesslightly elongated in a transverse direction and spaced apart a distanceslightly'greater than the space between the pins, sothat a-slight excessof plastic is disposed between the pins at the start of the formingoperation.

A sheet blank as above indicated is heated until soft enough to form.Oil heating is preferred to air heating because of the lubrication whichis obtainedbetweenthe plastic and the form, and the lesser tendencytoward dust marks on the finished product. As low a temperature ofheating aspracticable to permit forming is desired so-as to preventdistortion of the wired laminate prior to forming. We have found itsatisfactory to heat a blank three minutes at 240245 F.

After the blank is heated as above describedand while still coated withoil it is rapidly placed over the positioning pins 3 in the male mold A.The female saddle B is then pushed down 'overthe centering pins 9 intobending or forming engagement with the blank,and shortly after thesaddleB commences to'bend theshee't in a transverse direction of more or lessuniform curvature, a chin wiping memberC is moved downwardly, followingthe saddle B closely so that excess material is pushed out the sidebefore the saddle reaches the lower limit of its movement. Thisprocedure is critical in order to prevent side wrinkles in the product,but becomes less so as the amount of excess sheet plastic to beeliminated is reduced to a minimum by decreasing the form area in frontof the zone L, i. e., below the cut-off line defining the upper edge ofthe piece to be formed. When the saddle B has reached the bottom ofitsstroke (Fig. 3) it is clamped in position by means of clamps 11 andat that time the opposite edges of the plastic are disposed slightlyabove the base 1 in the zone L1; and at the somewhat smaller diameter ofthe form A (Fig.4) the lower faces of the legs 7 of the saddle B clampoppositely disposed and out wardly extending marginal edges of the blankfirmly against the base plateE. Concurrently with the movement of thesaddle B downwardly during the latter part of its stroke the wipingmember C is being moved downwardly and it is clamped (Fig. in itsdownward position by the clamps 21 simultaneously with the clamping ofthe saddle, or immediately thereafter. This serves to clamp theoutwardly extending marginal edges of the blank firmly against the baseplate E and holds the plastic in intimate contact with the rounded endof the form A.

In operating the wiping section or member D the panel 19 may be heldagainst the base plate E at the beginning of the wiping movement of thehinged plate 14'and we have found that satisfactory results may be thusobtained. However, this wiping action has also been performed by havingthe panel 19 spaced somewhat above the base plate E and simultaneouslymoving the panel 19 downwardly along its mounting pins 20 while swingingthe plate 14 in an anti-clockwise direction. In either mode of procedureit has been found convenient to mount the hinges on a separate panel topermit disassembly. However, it is to be understood that the hinges 18may be mounted directly on an integral part of the base plate E.

After the wiping member has been secured in clamped position the tuck-inmember D is moved by actuation of the lever 29 into the positionindicated in Fig. 8. This serves to exert pressure on the plasticadjacent the cutout portion 2 in a direction generally parallel to thebase plate E and to the initial position assumed by the blank at thebeginning of the operation. The pressure thus exerted on the zoneadjacent the cut-out portion 2 serves to sharply bend the materialinwardly at the chin section and exerts a tension on a contiguous areaof the plastic sheet.

After the formed blank has cooled the clamps are removed, the saddle B,wiping member C and tuck-in member D are withdrawn and the formed memberis removed and is ready for trimming along the outline designated ingeneral as P. In order to facilitate the removal of the face piece thepositioning pins 3 maybe slidably positioned in the form A with helicalcoil springs disposed between their lower end and bottom of the holes inwhich they are seated so as to permit the pins to be depressed below thelevel of the plastic.

From the foregoing it will be seen that the plastic blank S is initiallybent or formed transversely in a generally uniform curve and that whilethe transverse curvature in the zone L1 is being continued the remainderof the blank is being progressively bent or formed from the zone L1toward the smaller end of the form A and during such progressiveformation the blank is being formed or bent toward a longitudinal axisand simultaneously it is being formed or bent transversely about saidaxis. This serves to gradually move or flow the excess material downinto the marginal edge which is to be cut off and prevents wrinkles orundue distortion in the area of the piece to be formed.

Having thus described our invention with particularity with reference tothe presently preferred apparatus and in 6 connection with the preferredmethod, and having re ferred to certain modifications thereof, it'willbe obvious to those skilled in the art, after understanding ourinvention, that various other changes and modifications may be madetherein without departing from the spirit and scope of the invention,and we aim in the appended claims to cover such changes andmodifications as are within the scope of the invention.

What we claim is:

1. Apparatus for molding plastic hollow bodies from sheet blanks,comprising a male form provided adjacent one end with a portion ofrelatively uniform lateral curvature and tapering from said portionthroughout a longitudinally and laterally curved portion correspondingto the body to be formed, the smaller end of said, form being somewhatrounded in plan view, said form having a transversely extending base andbeing undercut to ma vide a relief zone below said tapered end, a femalesaddle form having a lateral curvature complementary to the curvature ofsaid male form portion at its larger end, means for guiding said femaleform for registered movement relative to said male form, a wiping platehaving a forward edge provided with a curved recess complemen-I tary tothe smaller rounded end of said male form, said wiping plate beingmounted for downward movement toward said male form and positioned sothat when moved downwardly to its lower limit its curved recess will liein spaced relation to said male die a distance substantially as great asthe thickness of the sheet blank being operated upon, and a tuck-inplate mounted for movement toward the rounded end of said male form insubstantial register with said relief zone and having a concave leadingedge adapted to be moved into contiguity with the perimeter of saidrounded end adjacent'said relief zone.

2. Apparatus for molding plastic face pieces for a helmet from sheetblanks, comprising a male form provided with a convex surface having alateral curvature and a longitudinal curvature and varying incross-sectional area from a larger to a smaller size, the smaller end ofsaid form being somewhat rounded in plan view, said form having atransversely extending base and being undercut at its smaller endadjacent said base, a female saddle form having a lateral curvaturecomplementary to the curvature of said male form at its larger end for alength substantially less than the length of said male form, meansmounting said female form for registered movement relative to said maleform in a direction toward and away from said base, a wiping platehaving a forward edge provided with a curved recess complementary to thesmaller rounded end of said male form, said wiping plate being mountedfor downward movement toward said male form and positioned so that whenmoved downwardly to its lower limit said forward edge will lie closelyadjacent the rear face of said saddle form and its curved recess willlie in spaced relation to said male die a distance substantially asgreat as the thickness of the sheet blank being operated upon, and atuck in plate mounted for movement toward the rounded end of said maleform in a direction in general alignment with said undercut portion andhaving a concave leading edge generally conforming to said rounded endof said male form and adapted to be moved into contiguity therewith.

3. Apparatus for molding plastic face pieces for a helmet from sheetblanks, comprising a laterally and longitudinally curved male form whosecross-sectional area varies longitudinally from a larger to a smallersize, the smaller end of said form being somewhat rounded in plan view,said form having a transversely extending base and being undercut at itssmaller end adjacent said base, a female saddle form having a lateralcurvature complementary to the curvature of said male form at its largerend for a length substantially less than the length of said male form,means mounting said female form r for registered movement relative tothe corresponding 7 portion of said male form, -a wiping plate having'afor- Ward edge providedwith a curved recess complementary to the/smallerro'undedend of saidmale-form, said wiping plate being hingedly mountedfor downward swinging movement toward said male form and positioned sothat When moved downwardly to its lower limit said curved recess willlie inspaced -relation to the corresponding surface of said male die adistance substantially as great as the thickness of the sheet blankbeingope'rated-upon, and a tuck-in plate mounted for longitudinalmovement towa'rdthe rounded end of said male form and having a concaveleading edge generally conforming to said rounded end and adapted to bemoved toward said roundedend in a direction in general alignment withsaid undercut portion'of said 'm'ale form.

'4. Apparatus for molding plastic face pieces for a helmet from sheetblanks, comprising a laterally and longitudinally curved male form whosecross-sectional area varies longitudinally from a larger to a smallersize, said form at its larger end being provided with a substantiallycylindrical segment and being of a depth greater than the depth of thearticle to be formed, the smaller end of said form being somewhatrounded in plan view, said'form having a transversely extending base andbeing undercut at its smaller end adjacent said base, afemale saddleform complementary to said cylindrical segment, means mounting saidfemale form for registered movement relative to the correspondingportion of said male form, a wiping plate having a forward edgeprovided'with a curved recess complementary to the smaller rounded endof said male form, said wiping plate being mounted for downward movementtoward said male formand positioned so that when moved downwardly to itslower limit said curved recess will lie in spaced relation to thecorresponding surface of said male die a distance substantially as greatas the thickness of the sheet blank being operated upon, and a tuck-inplate mounted for lon itudinalm'ovement toward therounded end of saidmale form and having a concave leading edge generally conforming to saidrounded end and-adapted to be moved toward said rounded end in adirection in general alignment with said undercut portion of said maleform.

5. Apparatus for molding plastic 'face pieces for a helmet from sheetblanks, comprising a laterally and longitudinally curved male form whosecross-sectional area varies longitudinally from a larger end to asmaller end, said form at its larger end being provided with asubstantially cylindrical segment extending beyond the extremity of thearticle to be formed and being of a depth greater than the depth of thearticle to be formed, the smaller end of said form being somewhatrounded in plan view, said form having a transversely extending base atsaid larger end and being undercut at its smaller end to adjacent saidbase, a female saddle form complementary to said cylindrical segment,means mounting said female form for registered movement relative to thecorresponding portion of said male form, a wiping plate havinga forwardedge provided with a curved recess complementary to the smaller roundedend of said male form, said wiping plate being mounted for downwardmovement toward said male form and positioned so that when moveddownwardly to its lower limit said curved recess will lie in spacedrelation to the corresponding surface of said male die a distancesubstantially as great as the thickness of the sheet blank beingoperated upon, and atuck-in plate mounted for longitudinal movementtoward the rounded end'of said male form and having a concave leadingedge generally conforming to said rounded end and adapted to be movedtoward said rounded end in a direction in general alignment with saidundercut portion of said male form.

'6. Apparatus for molding plastic face pieces for a helmet from sheetblanks, comprisinga male'form provided with-a convex surface having alateral curvature throughout a longitudinal distance in which thecrosssectional area varies fromia larger to a smaller-size, 'the smallerend'ofrsaid 'form being 'rounded in-plan view, said form at its'largerend having a transversely extendingbase secured :to la'supporting memberand being undercut at its smaller end Eto provide a relief zone adjacentsaid supporting member, a female saddle =form having a lateral curvaturecomplementary to thecurvature of said-male form at its 'largerenddisposed in vertical register with said base, vertical guide meansmounting said female form for registered movement relativeto said maleform, a wiping plate having a forwardedge provided with a curved recesscomplementary to the smaller rounded end of said male form, said wipingplate being mounted on said supporting member for downward swingingmovement toward said'male form and positioned so that when moveddownwardly to :its lower limit said forward edge willlie'closelytadja'cent Lthe rear-face of said saddle form and its curvedirecess will lie in spaced relation to the rounded end of 'said male diea distance substantially as greatas :the thickness 'of the sheet blankbeing operated upon,'a'tuck inplate having a concave leading edgecomplementary tothe recess in said male form, an actuating leverpivotally mounted on said supporting member and operably engageable withthe rear end of said tuck-in plate for urging the same 'toward'therounded end of said 'male form in :a direction generally parallel tosaid supporting member and in general alignment with said relief zone.

'7. The method of making a plastic face piece for a helmet whichcomprises heating a thermoplastic blank in an oil bathuntil softenoughto form, holding said blank against lateral and longitudinal movementrelative to a longitudinal axis,.par'tially bending portions of saidblank on opposite sides of said axissimultaneously in a transversedireetion aboutsaidaxis into a generally uniform curvature, continuingthetransverse bending of said blank into generally uniform curvaturethroughout an end zone extending a minor portion of its length andsimultaneously therewith exerting a wiping pressure on the remaininglength of said blank beyond said zone and thereby bending said remaininglength in a transverse and longitudinal direction into a taperedcurvature of gradually decreasing cross-sectional dimensions, holdingsaid partially formed blank while restraining its curved surfaceinteriorly throughout :its tapered extent, and while so held andrestrained exerting pressure on the peripheral edge portion of saidpartially formed blank at-the end of said tapered curvature in adirection toward said end zone and generally parallel to saidlongitudinal axis to thereby form a rounded end.

8. The method of making a plastic face piecefor a helmet which comprisesheating'a thermoplastic blank of a length and width greater than thecorresponding surface dimensions of the face piece to be formed untilsoft enough to form, partiallybending portions of said blank onopposite-sides of said axis simultaneously throughout its entire lengthin 'at'ransverse direction about -:a longitudinal axis, continuing thetransverse bending throughout a limited end zone "disposed beyond thearea of the face piece to be formed until said end zone conforms to agenerally uniform curvature and during said continued bending in saidend zone simultaneously bending without appreciable stretching'theremainder of said blank longitudinally toward and in a transversedirection around said longitudinal axis to provide a rounded andgradually tapering curved surface corresponding to the major area of theface piece to be'formed,-se'curely holding said partially formed blankwhile restraining its curved surface interiorly throughout its taperedextent, and while so held and restrained exerting pressure on theperipheral edge portion of said partially formed blank at the end ofsaid tapered curvature 'in' a direction toward saidend zone andgenerally parallel to said longitudinal axis to thereby form a roundedend. I

9. Method of making a hollow plastic body generally tapered in sideelevation and plan view which comprises heating a thermoplastic blankuntil soft enough to form, placing the softened blank over a male formhaving the desired contour of the finished article, bending respectivelyoppositely extending side portions of said blank simultaneously oversaid male form in a transverse direction until said blank lies closelyagainst said form throughout an end zone extending for a minor portionof its length, after said blank has been only partially bent andconcurrently with the further formation of said end zone progressivelyforming without appreciable stretching the major portion of said blankin a transverse and longitudinal direction against said male form,holding said partially formed blank firmly against said form, and whileso held exerting pressure on the marginal lower portion of saidpartially formed blank in a direction generally parallel to the blankposition before the first bending operation.

10. Method of making a hollow plastic face piece having a doublecurvature and being of generally semi-elliptical outline which comprisesheating a thermoplastic blank in an oil bath until soft enough to form,placing the softened blank coated with oil over a male form having thedesired contour of the finished article and having an extended end zoneof generally semi-cylindrical shape, bending respectively oppositelyextending side portions of said blank simultaneously over said male formin a transverse direction until said blank lies closely adjacent saidform throughout said end zone, after said bending has been onlypartially performed and concurrently with its completion progressivelyforming without appreciable stretching the area of said blankcorresponding to said desired contour in a transverse and simultaneouslyin a longitudinal direction against said male form, holding saidpartially formed blank firmly against said form, and while so heldexerting pressure on the marginal lower portion of said partially formedblank in a direction generally parallel to the blank position before thefirst bending operation to thereby urge the same closely against the endof said form.

References Cited in the file of this patent UNITED STATES PATENTS

